The bracketed information herein refers to documents listed at the end of this document, which are referred to by reference number, author, and year of publication.
Ultrasonic ranging systems have been widely used in applications such as mobile robots, surface/subsurface feature detection, medical imaging, etc. [Manthey, 1992 (R12)] [Ashley, 2001 (R3)]. The primary performance factors of such ranging systems include: resolution, power, and scattering. Since phase information is inherent in ultrasound beam propagation and beam formation, to achieve high performance, it is preferable to transmit short duration acoustic pulses such that the phase interference is minimized. However, due to the narrow transducer bandwidth, an ultrasonic transmitter driven by a conventional Rectangular Modulation Pulse (RMP) drive signal tends to output signal energy that includes transient acoustic waves of significant duration and amplitude, which degrade system performance significantly.
Low Transient Pulse (LTP) signals, as opposed to the RMP acoustic signals, have shorter transient pulse durations and thus incur less phase interference and thereby experience better performance. Providing a prescribed signal to drive the transmitter may enable production of signals with low transient pulse energy.
A number of methods have been proposed in connection with the design of a prescribed excitation signal. Zero-phase cosine-magnitude pulses [Berkhout, 1984 (R4)] and [Cobo, 1993 (R6)] are optimally synthesized for high resolution acoustic imaging applications. Such drive signals are synthesized by the inversion of the transducer transfer function and the prescribed transmitted acoustic signal spectrum. The bionic pulse method [Altes, 1975 (R1)] [Altes, 1976 (R2)] [Skinner, 1977 (R17)] minimizes the frequency and amplitude variations caused by Doppler effects. Such pulses are aimed at Doppler tolerant applications, and the drive signal is generated by the inversion of the transmitter transfer function and the defined bionic spectrum so that the Doppler effects on acoustic signals are minimized. The weighted least-squares filter [Mandersson, 1989 (R11)] is a least-squares filter modified by introducing a weighting function that defines the resolution of an ultrasonic transducer. To pre-shape the drive signal, the filter impulse response is evaluated by minimizing the weighting error between the received acoustic echo signal corrupted with noise and the desired acoustic echo signal such that the duration of the ultrasonic echoes is decreased. Parametric shaping deconvolution [Cobo, 1995 (R7)] is an approach that generates the desired drive signal by inverting the transmitter transfer function and the desired acoustic waveform with a suitably selected regularization parameter, which provides stabilization and results in a narrower acoustic pulse transmitted at the cost of the amplitude loss. The transducer transient suppression [Piquette, 1992 (R13), (R14)] is a method for suppressing the transients of the transmitted acoustic pulses where the transient-suppressing drive signal is approximated using a backward solution of the equivalent LCR circuit of a spherical transducer. In [Piquette, 1996 (R15)], an exact expression of the transient-suppressing drive signal for an equivalent circuit containing exclusively lumped passive elements is further derived by introducing the Heaviside unit step function to describe the desired transient-suppressing circuit output. The numerical realization of such transient-suppressing drive is obtained by adopting an approximation to the delta function. [Piquette, 1996 (R15)] also addresses an approach to generating an approximate transient-suppressing drive by inverting the transducer discrete-time transfer function and the desired transmitted acoustic signal. Besides preshaping the drive signals, algorithms of the time of flight estimation are usually used to enhance the resolution in acoustic ranging systems. For example, the multiecho parameter estimation for acoustic ranging systems [Li, 2003 (RI0)] is a time-of-flight estimation algorithm for the joint proximity range estimation and the secondary echo mitigation.
There is a need in the art for a system and method for sonic wave based measurement that reduces transient energy in transmitted waves to levels lower than that available in existing systems.